Diabroticine beetles are a significant problem during the growth of, inter alia, corn (field, pop, seed, and sweet), beans, Cucurbitaceae (including cucumbers, melons, squash, and pumpkins), peanuts, peas, potatoes, and sweet potatoes. Corn is conveniently used to describe the effects of diabroticine beetles. These pests are the direct or indirect (i.e., as a vector for bacteria and inoculation of melons and squash) cause of millions of dollars of crop and garden damage annually. Damage by these beetles has continued despite over 30 years of attempts at control.
Diabroticine beetles encompass multivoltine and univoltine species. Multivoltine species (e.g., the southern corn rootworm) can produce up to 3 generations a year. Univoltine species (e.g., northern and western corn rootworm) have a life cycle that starts with eggs laid 4-24 inches below the soil in the Fall. In early Spring and over the course of several weeks, the larvae hatch and begin to feed on nearby roots thereby destroying the root's anchoring abilities and the microhairs responsible for mineral, nutrients, and water assimilation. If the plant roots have not been so damaged that the plants falls over, the yield from the affected plants is reduced due to impaired nutrition.
After feeding, the diabroticine larvae pupate and emerge from the ground as adult beetles. Univoltine beetles emerge at some time during mid July through August (depending on local climate). Male diabroticine beetles emerge about 1 week before the females (week 1) which, in turn, emerge at about the same time as corn silks emerge. Because the fresh silks emit a number of volatile agents which are attractive to both the male and female beetles, the 7-10 days of silking represents a period of high feeding activity for the beetles. The beetles immediately begin to migrate up the stalk toward the leaves, ears, and silks. This compulsion is quite strong since there is evidence that the beetles will not move down the corn stalk in response to attractants. Throughout this period, the beetles feed and mate.
The key to control of the diabroticine beetles is to disrupt the life cycle by affecting the larvae and/or adult beetles. One method known in the art as "banding" refers to the practice of trying to control the larvae by applying a contact insecticide in or along a furrow containing planted seeds. The theory behind banding is that larvae will enter the treated area when searching for roots and die due to contact with the insecticide.
Unfortunately, microbial attack impairs the efficacy of the insecticides well before all the larvae have had time to hatch and, enter the band. Additionally, concerns for groundwater contamination. The impact on nontarget organisms (e.g., bird kill), and the hazards of human exposure to the toxic insecticides restrict the use of insecticides that might survive the effects of microbial attack during the larval feeding stage. Moreover, plant roots often extend well beyond the band leaving the roots vulnerable to attack.
Recently, it has been proposed to use the tissue of dried gourds from the Cucurbitale order in combination with 0.01-10% by weight of an insecticide to make a lethal bait for the control of diabroticine beetles. From Canadian Patent No. 1,195,922, the bitter tasting gourd tissue acts as a compulsive feeding stimulant for diabroticine beetles but does not harm beneficial insects. By coating the gourd tissues with an insecticide, the beetles compulsively consume a lethal quantity of insecticide. From the examples in the patent, gourd fruits were dried and ground in a food processing mill to 30-60 mesh (250-550 micron diameter), surface coated with an insecticide solution, and broadcast over the tops of sweetcorn plants at 10-100 lbs. per acre. The bait was reported to be effective at killing diabroticine beetles for at least two weeks. Later developments in U.S. Pat. No. 4,880,624 reported an increased level of efficacy when the composition is mixed with volatile attractants.
Notwithstanding the promising results obtained by the use of a compulsive feeding stimulant, there remains room for even higher levels of diabroticine control.